Porous inorganic oxides having uniform mesopores, which are synthesized by utilizing self-organization of an organic compound and an inorganic compound, are known to have a large pore volume, a large surface area, or the like, as compared with conventional porous inorganic oxides such as zeolite. Application of these porous inorganic oxides to a catalyst carrier, a separation absorbent, a fuel cell, a sensor, or the like has been investigated, for example, in the form of a film.
One problem in using a porous silica film, which is one of the oxides having uniform mesopores, for optical functional materials, electronic functional materials and the like, especially for semiconductor interlayer insulating films, is how to satisfy both the porosity and the mechanical strength of the film. Specifically, when the porosity of the film is increased, the density of the film is decreased. As a result, the relative dielectric constant of the film is decreased to come close to 1, which is a relative dielectric constant of air. On the other hand, when the porosity is increased, internal spaces are increased and the mechanical strength is lowered to a considerable degree.
Further, since the mesopores of the porous silica have a significantly large surface area and have silanol (Si—OH) groups on the surface thereof, H2O, which has a high relative dielectric constant, is easily adsorbed thereto. Accordingly, there is a problem in that the relative dielectric constant, which has been lowered by increasing the porosity, is increased to the contrary as a result of increasing the adsorption.
As a method for preventing the adsorption of H2O, a method is proposed in which a hydrophobic functional group is introduced into a film, for example, a method in which the adsorption of water is prevented by trimethylsilylating the silanol groups in the pores (see the pamphlet of International Publication WO00/39028).
In addition, it is reported that not only the hydrophobic property but also the mechanical strength of a porous film composed of a Si—O bond can be improved by allowing the same to contact a cyclic siloxane compound in the absence of a metal catalyst (see the pamphlet of International Publication WO2004/026765).
These methods achieve improvements in not only the hydrophobic property but also the mechanical strength. However, since these methods require a special apparatus for treating a porous film with a cyclic siloxane compound or facilities for disposing of an exhaust gas, there has been demand for a simpler treatment method.
Further, there is a report in which an alkali metal compound is included in a silica-based film-forming composition for the purpose of improving the performance thereof by the addition of an additive (see Japanese Patent Application Laid-Open No. 2006-291107). It is also reported that examples of the alkali metal include sodium, lithium, potassium, rubidium and cesium, and that the inclusion of these alkali metal compounds not only lowers the dielectric constant of a silica-based film formed from the silica-based film-forming composition but also improves the mechanical strength thereof and, moreover, improves the storage stability of the porous film-forming composition.
However, in the fields in which even higher accuracy is required in controlling the physical properties, such as the field of a semiconductor, there is a need for higher storage stability of the porous film-forming composition as a raw material for a porous film.
As described above, even with improvements in technologies for producing a porous film suitably used for optical functional materials, electronic functional materials and the like, and proposals of methods for lowering the relative dielectric constant or performing modification in order to increase the mechanical strength, a technique for producing a porous film that satisfies an even lower relative dielectric constant and an even higher mechanical strength, and a technique for producing a porous film-forming composition that exhibits an even improved storage stability, have yet to be established.